The valves with which we are concerned here are used to throttle the flow of fluids maintained at very high temperatures (e.g. 850.degree. F.) and pressures (e.g. 2000 psi) and containing abrasive particles, a coal ash slurry for example. In these difficult service applications, the valve parts suffer a considerable amount of erosion and wear. The conventional way to maximize the service life of such valves is by fabricating those valve parts which are particularly prone to erosion of very abrasive resistant materials and specially profiling and streamlining them to achieve smooth fluid flow around those parts. However, such parts made of those hard materials are extremely difficult to fabricate and are therefore quite costly. As a practical matter, then, when designing such valves there is inevitably a compromise made between valve cost and service life.